Kolo ol Bacteria in I'^'xation Fiocrss 



211 



these are resp()n.sil)le for nitrogen fixation, W'oronin, who also lonnd 

 in 1866 that the nodnles eonsist of bacteVia, eonsidered the iKxhilcs 

 pathologieal ontgrowths. In 1879, Frank demonstrated that th(> 

 formation of nochiles ean he prevented 1)\- sterih/ation ol soil; he 

 snggested that the nodules are eansed in ontside infeetion. 



Atwater and ^^'oods, working in C'onneetieut, reeogni/cni in 1884 

 the possihilitx that both plants and baeteria are faetors in the process 

 of fixation of atmospheric nitrogen. Soon afterward, Hellriegel and 

 ^^'i]farth in German) demonstrated tliat the nodules on the roots of 

 k'guminons plants are due to bacterial infection, and that this is a 



Fig. 88. Life histon' of root-nodule bacteria in the tissue of the alfalfa nodule 



( from Thornton ). 



beneficial process, since within these nodules the bacteria fix the 

 atmospheric nitrogen. Plants could be grown on artificial soils 

 containing only traces of combined nitrogen, provided the mineral 

 elements necessary for the nutrition of the plant were present and 

 nodules were formed. In absence of nodules, the plants were unable 

 to utilize the atmospheric nitrogen for their growth. When sterilized 

 soil was treated with a suspension of fresh soil, nodules were formed 

 and the plants grew normally. The growth of the cereals depended, 

 however, on the nitrate content of the soil. These results were soon 

 confirmed by Lawes and Gilbert and others. 



The causative organism responsible for nitrogen fixation in pure 

 culture was isolated in 1888 by Beijerinck, who named it Bacillus 

 radicicola. He described three stages in the development of the 

 organism : 



1. The bacterium is present in the soil in the form of small rods 

 which can penetrate the root hairs of the leguminous plants and 

 from there are transferred to the "infectious tissue." 



2. The organism later changes into a motile form. 



3. Within the plant tissues, the cells are transformed into bac- 

 teroids, which function as the symbiotic mechanism. 



